1. Field of the Invention
The present invention relates to compositions and methods for drilling boreholes in subterranean formations, particularly hydrocarbon bearing formations. More particularly, the present invention relates to oil or synthetic based drilling fluids, fluids comprising invert emulsions, and more particularly to drilling fluids utilizing esters which combine high ecological compatibility with good stability and performance properties.
2. Description of Relevant Art
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation.
Oil or synthetic-based muds are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite or other evaporite formations, hydrogen sulfide-containing formations, and hot (greater than about 300 degrees Fahrenheit) holes, but may be used in other holes penetrating a subterranean formation as well. Unless indicated otherwise, the terms “oil mud” or “oil-based mud or drilling fluid” shall be understood to include synthetic as well as natural or traditional oils, and such oils shall be understood to comprise invert emulsions.
Oil-based muds used in drilling typically comprise: abase oil (or synthetic) comprising the external phase of an invert emulsion; a saline, aqueous solution (typically a solution comprising about 30% calcium chloride and emulsifiers) comprising the internal phase of the invert emulsion; and other agents or additives for suspension, weight or density, oil-wetting, fluid loss or filtration control, and rheology control. An oil-based drilling fluid may commonly comprise between about 50:50 to about 95:5 by volume oil phase to water phase. An all oil mud simply comprises 100% oil by volume; that is, there is no aqueous internal phase.
Oil-based muds or drilling fluids comprise a key segment of the drilling fluids industry. However, increasingly oil-based drilling fluids have been subjected to greater environmental restrictions and performance and cost demands. There is consequently an increasing need and industry-wide interest in new low-cost drilling fluids that provide robust rheology at high and low temperatures simultaneously while still affording environmental acceptance.